The present invention relates to the casting of metals in steel casting dies. In industrial die casting processes, molten metal is injected into a cavity formed by two sections of a steel casting die wherein the molten metal is allowed to solidify into a cast metal piece. The die sections, one stationary and one movable, contain tubes for circulating water to cool the sections in order to accelerate the solidification of the molten metal. After the molten metal has solidified, the steel casting die sections are separated and the cast metal piece is ejected.
Sometimes, molten metal freezes too rapidly, especially in thin sections of the casting. One of the undesirable effects caused by molten metals freezing too rapidly in the die cavity is the problem of "cold shuts" (i.e., obstruction of the flow of molten metal in the die by premature solidification, usually in the thin section of the casting) in which the metal solidifies prematurely thereby preventing complete filling of the die cavity. Also the metal may exhibit a patterned, rather than a smooth, surface on the piece. After extended use, the elevated temperatures and pressures involved in die casting processes can damage the steel casting dies and cause surface imperfections in the cast piece. In addition, fusion or sticking of the molten metal to the steel casting die surface can occur unless the die surface is first treated with a lubricant, antisticking agent or mold release composition.
As reported in Metals Handbook, the production of castings having thin sections presents problems in many cases because of temperature considerations. The temperature at which a die will operate during continuous operation depends on the weight of the "shot," the surface area of the shot, the cycle speed, and the shape of the die. When dies are too cold, cold shuts, laminations, internal incomplete filling and poor finishes with excessive flow marks are likely to result.
Cold shuts or flow marks in zinc alloy die casting also occur when the sprays of molten metal that first contact the surface of the die harden and are not remelted by the heat of the metal that later enters and fills the die cavity. The sprayed metal shows on the surface of the casting as laps or layers or as partly melted pieces embedded in the casting, or the cast part is incompletely formed due to incomplete filling of the die.
Some casting shapes require localized heating above the established temperature. Metal overflows are often used to heat die areas surrounding peripheries having thin casting sections far from the main runner. This method of local heating helps to fill thin casting sections and to improve casting finish. In the past, cold shuts have been eliminated by decreasing the cycle time to increase the die temperature; or by reducing coolant flow in the area of the die corresponding to the affected area of the casting; or by heating the die with external heaters.
Depending upon the particular conditions, the minimum thickness that can be produced in a zinc die casting is of the order of about 0.25 mm., usually around 0.5 mm. to 0.8 mm. As suggested above, the minimum thickness which can be tolerated is also a function of the physical size of the casting and other variables as well. It is castings of such thin sections to which one object of the present invention is directed because the thermal insulation provided by the die coatings of the present invention permits the use of thin casting sections while still avoiding the disadvantages of premature solidification of the molten metal, and avoiding the need for unusual measures such as extra heaters or die overflowing to avoid the cold shuts, etc. For example, use of the present invention permits the flow of molten metal into areas having constricted cross-sections, such as automotive radiator grills.
In the past, boron nitride has been used as a lubricant for industrial machinery because of its high temperature stability and thermal insulation properties. When used alone, however, boron nitride does not adhere effectively to the die surface, and it has been necessary to disperse it as a powder in a suitable carrier. In this regard, U.S. Pat. No. 2,726,160 describes a composition of boron nitride in water using an acetate of an acylated diamine as a dispersing agent. According to the patent, the composition is applied as a thin coating on casting molds for glass and metal. The coating is not expected to provide continuous protection for the mold and must be reapplied every several hours.
Various polymers have also been used as lubricants in the die casting of metals. When used alone, many polymers decompose, emit noxious fumes, or become tacky upon contact with molten metals. U.S. Pat. No. 2,923,041 relates to the treatment of metal casting molds for nonferrous metals using low molecular weight hydrocarbon polymers, such as polyisobutylenes and polyethylene. Alpha-olefinic polymers in organic diluents, including ketones, toluene and benzene, are disclosed in U.S. Pat. No. 3,253,932. Polyol esters of fatty acids are described as liquid lubricants in U.S. Pat. No. 3,620,290.
Other known coating compositions for coating metal casting molds include molybdenum disulfide in a polymer base as described in U.S. Pat. No. 3,447,588. A mixture of graphite, cryolite and a boron-containing compound for protecting the surface of a metal is described in U.S. Pat. No. 3,685,986. The boron-containing compound is either boron anhydride or boron ore. Casting molds containing silicon nitride are described in U.S. Pat. No. 3,709,459. In U.S. Pat. No. 3,727,666, a thin refractory composition, which may contain up to 5% of a water-based latex is used to prevent contamination of the molten metal and sticking of the metal at the casting mold surface. Finally, a coating of an inorganic binder and a metallic material or a solid lubricant for treating die surfaces is disclosed in U.S. Pat. No. 3,895,899. The patent describes the formation of a layer of nitride and borides as diffusion products which result from the simultaneous penetration of nitrogen and boron into the working surfaces of the mold.
Moreover, the foregoing prior art does not suggest any solution to the problem of premature solidification or "cold shuts" in die casting.
In the copending application of Schrade F. Radtke, Ser. No. 857,939, filed Dec. 6, 1977, which is incorporated herein by reference, there is disclosed and claimed a method for treating steel casting dies wherein a composition of boron nitride in an elastomeric polymer and an organic solvent is used. However, the use of organic solvents in a casting machine may create problems of toxic fumes and flammability (plust cost) which would require special precautions or special apparatus.